1. Field of the Invention
The present invention relates to a porous substrate for epitaxial growth, a method for manufacturing the same, and a method for manufacturing III-nitride semiconductor substrate, and particularly to a porous substrate for epitaxial growth by which III-nitride semiconductor substrate having a low defect density can be epitaxially grown in good reproducibility, a method for manufacturing the same, and a method for manufacturing III-nitride semiconductor substrate.
2. Description of the Related Art
GaN compound semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN), and gallium aluminum nitride (GaAlN) are significantly watched as materials for optical device such as blue light emitting diode (LED), and laser diode (LD). In addition, since GaN compound semiconductors are excellent in heat resistance and environmental resistance, a development for applying the GaN compound semiconductors having such remarkable characteristic features to elements for electronic device is started.
Bulk crystal growth of GaN compound semiconductors is difficult. In this connection, a GaN free-standing substrate which can be served for practical use is in the midway of development. At present, sapphire is widely used for a substrate for GaN growth. A method for epitaxially growing GaN on a monocrystalline sapphire substrate by means of metal organic vapor phase epitaxy (MOVPE) technique or the like is commonly used.
However, since a sapphire substrate has a different lattice constant from that of GaN, a monocrystalline film cannot be grown, when the GaN is directly grown on the sapphire substrate. For this reason, there is proposed a method wherein an AlN or GaN buffer layer is once grown on a sapphire substrate at a low temperature to moderate distortion in lattice by means of the low-temperature growth buffer layer, and then GaN is grown thereon (for example, Japanese patent application laid-open No. 1988-188983). When such a low-temperature growth nitride is used as a buffer layer, monocrystalline epitaxial growth of GaN is possible. In even this method, however, a discrepancy in lattices of the substrate and the crystal is a problem. The resulting GaN has a dislocation density of 109 to 1010 cm−2. This defect is an obstacle in manufacturing GaN LDs.
Growth techniques such as ELO [Appln. Phys. Lett. 71 (18), (1997), pp 2638 to 2640], FIERO [Jpn. J. Appln. Phys. Vol. 38 (1999), pp L184 to L186], and pendeo-epitaxy [MRS Internet J. Nitride Semicond. Res. 4S1, G3. 38 (1999)] are reported as a method for reducing defect density resulted from a difference in lattice constants of sapphire and GaN. These growth techniques are such that a mask which is patterned with SiO2 or the like is grown on GaN grown on a sapphire or the like substrate, GaN crystal is selectively grown further through window openings of the mask to cover the mask with the GaN by means of lateral growth, whereby propagation of dislocation from underlying crystal is prevented. Because of a development of such growth techniques, it becomes possible to reduce remarkably a dislocation density in GaN to an order of around 107 cm−2. An example of the techniques is disclosed in Japanese patent application laid-open No. 1998-312971
Although GaN crystal having a low dislocation density can be obtained in accordance with these ELO techniques, the GaN crystal is grown on a heterogeneous substrate such as sapphire. Accordingly, a method for removing such heterogeneous substrate is required separately to obtain a free-standing substrate.
As a simple method for removing a heterogeneous substrate, the present applicant has proposed an exfoliation method by void formation wherein a Ti thin film is grown on a sapphire substrate on the surface of which a thin GaN layer is grown, the resulting sapphire substrate is heat-treated in a mixed gas stream of, for example, hydrogen and ammonia to obtain a porous substrate containing voids, a thick GaN film is grown on the porous substrate, and after the growth, the thick GaN film is easily exfoliated from the substrate by means of behavior of voids in the growth interface (Japanese patent application No. 2001-090148).
In the above-described exfoliation method by void formation, a flat GaN layer having thin thickness of around 300 to 500 nm is desirable. When a thick GaN layer is used, voids to be formed in the layer become very deep, so that another thick GaN film to be grown thereon is difficult to obtain a mirror surface and to achieve exfoliation.
On the other hand, it is not so easy to grow a thin and flat GaN layer on a sapphire substrate in good reproducibility. Even a slight change in a condition of a growth furnace, by which though growth in a comparatively thick film is scarcely affected adversely, influences remarkably a state of crystal in a thin film, resulting in difficulty to make the surface of the film flat. Accordingly, there is such a problem that a growth condition must be checked each time. Even if there is a flat appearance, unevenness in crystallinity is significant, so that a heat-treating condition to form voids is not constant, resulting in necessity for checking the heat-treating condition each time.